The present invention relates to a mixer.
Mixers have been used in radio circuits for a long time. A mixer may be used to produce an intermediate frequency (IF) signal dependent on a received radio frequency (RF) signal and a local oscillator (LO) signal. The IF signal is delivered to a detector circuit, which generates an audio frequency (AF) signal in response to the IF signal. Hence, the mixer in co-operation with a suitable detector circuit can operate to extract a message from a radio signal.
U.S. Pat. No. 5,589,791 to Gilbert describes a classic active mixer commonly known as the xe2x80x9cGilbert mixerxe2x80x9d. The Gilbert mixer comprises a mixer core having four transistors whose bases are connected to an LO port for receiving a LO signal and whose collectors are connected to an IF output port. The mixer also has an RF input section having two transistors whose bases are connected to an RF port for receiving an RF signal and whose collectors are connected to the emitters of the transistors in the mixer core.
The operation of the mixer is as follows: In the absence of any voltage difference between the bases of the two transistors in the RF input section, the collector currents of these two transistors are essentially equal. Thus, a voltage applied to the LO port results in no change of output current. Conversely, if an RF signal is applied to the RF port, but no voltage difference is applied to the LO port, the output currents will again be balanced. Thus, it is only when a signal is applied to both the LO port and the RF port that an intermediate frequency signal appears at the IF port. A known problem with the classic active mixer is the switching noise generated by the mixer core transistors as they switch between their xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states. U.S. Pat. No. 5,589,791 to Gilbert describes this problem, and illustrates noise bursts that are created during transition periods when the LO signal changes between high and low states. U.S. Pat. No. 5,589,791 to Gilbert also discloses a mixer having an RF input port, an LO input port and an active input driver connected to the LO input port. The input driver is a complex circuit including an input for receiving a single-sided LO signal, and no less than 21 transistors forming class AB emitter followers and an associated bias stage. The active input driver aims to cause a forced supply and withdrawal of charge from each of the LO input terminals of the mixer core for providing quicker transitions between the on and off states in the mixer core.
An aspect of the invention relates to the problem of providing a mixer with improved performance at low cost.
This problem is addressed by a mixer, comprising:
a first terminal and a second terminal forming a first input port for receiving a first signal having a first frequency;
an second input port for receiving a second signal having a second frequency;
a mixer output port for a resulting signal; and
a first group of valves having their control inputs coupled to the first terminal for receiving the first signal;
a second group of valves having their control inputs coupled to the second terminal for receiving the first signal; and
a third group of two valves having their control inputs coupled for receiving the second signal;
said valves co-operating such that in operation the mixer produces the resulting signal responsive to the first and second signals. Moreover, the mixer comprises at least one passive low pass filter having an inductor; said at least one low pass filter being connected to the control input of a valve.
This filter advantageously operates to decrease the rise time of the signal controlling the valve. A quicker rise time of that signal causes a quicker transition of the valve from non-conducting to conducting state. Since the noise produced by a mixer emanates to a large extent from noise produced by a valve during transition from a conducting state to a non-conducting state, the total amount of noise is thereby reduced, when the mixer is provided with such filters. Since the filter has only passive components, the reliability of the mixer is improved and the noise contribution is minimal. An additional advantage attained with passive components is a low component cost. Hence, the mixer, when used in a receiver renders a reliable high fidelity radio receiver at a low cost.